High surface area chromia aerogel efficient catalyst and catalyst support for ethylacetate combustion

摘要

The effect of dispersion and ensembling mode of chromium oxide nanocrystals in bulk xerogels and aerogels on their performance as well as efficiency of high surface area chromia aerogel as catalysts support were investigated in complete ethylacetate (EA) oxidation. Two series of chromia catalysts with a structure of alpha-Cr2O3 (7-110 m(2) g(-1)) and alpha-CrOOH (230-735 m(2) g(-1)) were tested in EA complete oxidation as a model reaction for VOC combustion. For alpha-Cr2O3, the specific activity increased 3.3 times with decreasing the crystal diameter from similar to100 to 13 nm. For alpha-CrOOH materials, where the surface area was determined by the different packing mode of primary nanoparticles of the same size (3-5 nm), the similar specific rate constants were measured with all the tested samples. The activity of the chromia aerogel (alpha-CrOOH, 630 m(2) g(-1)) was four times higher compared with 0.5% Pt/Al2O3 and 30 times higher relative to 30% Cr2O3/SiO2. Oxidative treatment (O-2) at elevated temperatures converts both phases to CrO2, in case of alpha-Cr2O3-only in the crystals surface layer. In both reduced and oxidized states, high concentration of surface oxygen vacancies were detected in alpha-Cr2O3 and alpha-CrOOH catalysts. A redox cycle Cr(III) []OH <----> 4 Cr(IV)[O]O which determines the catalysts performance at the surface of both types of bulk chromia materials was proposed. Promotion of high surface area chromia aerogel with Pt, Au, Mn and Ce increased its activity in EA complete oxidation by a factor of 1.25-2.7. Addition of Pt, Mn to ceria-promoted chromia aerogel has a significant effect, yielding a high specific rate constant. Promotion with Ce- and Mn-additives improved the efficiency of redox cycle in Cr-aerogel and increased the concentration of surface oxygen vacancies. (C) 2003 Elsevier B.V. All rights reserved. [References: 39]